Clo2 depilation and kerating removal from hides and skins



United States Patent 3,294.479 ClO DEPILATION AND KERATIN REMOVAL FROM HIDES AND SKINS. Kurt Rosenbusch, Frankfurt am Main, Germany, assignor to Farbwerke Hoechst Aktiengesellsbbaft vormals 5 5 Claims. (51. 8-94.12)

This is a continuation-in-part of the copending application Ser. No. 388,932. filed August 11, 1964.

The present invention provides a new process for removing the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts, for example hairs or scales.

By unhairing, there is understood in the tanning industry an operation in which the hair of hides and pelts is loosened to such an extent that it can easily be mechanically removed from the dermis together with the keratin-containing parts of the hides such as the dermis, hairs, and scales.

Processes have been proposed in which loosening of the hair is effected by bacterial or enzymatic action. In other proposed methods, for example, in the so-called chemical liming processes, strong alkalies, for example calcium hydroxide, soda lye, or low molecular weight alkylamines are used, at d inorganic sulfides or hydroxides, in most cases sodium sulfide or arsenic sulfide, are added as so-called priming agents.

Bacterial or enzymatic unhairing processes have the disadvantage that the short undercoat of the hair and the roots are often not affected; these spotted, incompletely dehaired pelts thus obtained must be subjected to a second chemical unhairing process. .The chemical liming process has the disadvantage that the hides are considerably swelled by the alkalies used, and this af fccts the sensitive grain. Because of this swelling, the finished leather shows protruding fat-wrinkles, and looseness of the grain of the finished leather caused by the movement imparted to the swollen hides during dressing is often observed.

In the liming process using lime sulfide, which at present is the most widely used process, the lime must be removed from the hides by an additional operation, because the lime forms precipitates with almost all leather treating auxiliaries and this impedes the penetration of the auxiliary agent into the hide. Further disadvantages in the use of sulfides are theconsiderable difficulties involved in removing them from wastes, which is often required, and the disagreeable odor of the hydrogen sulfide formed.

Now, it has been found that the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts can be easily removed in a particularly advantageous manner when the hides and pelts are exposed to the action of chlorine dioxide. This chlorine dioxide can be contacted with wet, preferably soaked hides and pelts in the form of a gas, suitably with exclusion of oxygen to prevent the risk of an explosion, and conveniently in admixture with an inert gas such as carbon dioxide or nitrogen. In practice, however, it is preferred to contact the skins and hides with chlorine dioxide in an aqueous medium at a goods to liquor ratio of about 10 to 500% (referred to the weight of the soaked skins and hides) as is usual in the known chemical liming process.

To practice the process of the present invention, the chlorine dioxide gas may be introduced into an aqueous medium containing the hides to be treated. The chlorine dioxide gas can be easily produced in a suitable apparatus by, for example, acidification of an alkali metal chlorite solution, by reduction of chloric acid or its salts by means of a reducing agent such as sulfurous acid or oxalic acid, or .by the action of concentrated sulfuric acid on an alkali metal chlorate. The production of chlorine dioxide gas from chloric acid and oxalic acid is very advantageous because carbon dioxide is formed as an oxidation product of oxalic acid and dilutes the explosive gaseous chlorine dioxide so that it can be handled without risk. The chlorine dioxide gas stream is introduced into the unhairing bath through a distributing tube, if desired together with an inert gas, for example nitrogen or carbon dioxide, as a diluent. The unhairing bath should have a pH-value below about 7.8, and preferably should not be alkaline, to prevent salt formation, for example the formation of alkali metal chlorites and chlorates. With a view to better handling, it is preferable in many cases to use aqueous solutions of substances which give off chlorine dioxide, instead of gaseous chlorine dioxide, for unhairing in an aqueous'medium. As such substances which give off chlorine dioxide, the watersoluble salts of chlorous acid, especially the alkali metal chlorites, are especially suitable. For reasons of economy, sodium or potassium chlorite is preferred. Aqueous solutions of these substances give off chlorine dioxide upon acidification, for example at pH-values below 7.8, preferably below pH 7. Therefore, if it is not intended to use chlorine dioxide 'gas in the unhairing process according to the present invention, aqueous acidic solutions of substances that give off chlorine dioxide are generally used at pH-values below pH 7.8.

In this method of operation, it is preferable to use a weakly acid medium having -a pH-value in the range of pH 3 and pH 6. to ensure a slow and uniform evolution of chlorine dioxide and a long lasting action of the chlorine dioxide on the hides and pelts to be treated. In principle, however, it is also possible to adjust the pH of the unhairing solutions to acid pl-l-values outside the mentioned range, for example. to a value of below pH 3 or between pH 6 and 7.8. In the latter case, the pH is inherently reduced in the course of the treatment by the hydrochloric acid and chloric acid formed. The use of pH-values below pH 3 involves the risk that the evolution of chlorine dioxide may proceed too violently so that the chlorine dioxide escapes too rapidly and is partly unused.

For adjusting the pH-value, any mineral or organic acids which are inert towards chlorine dioxide may be used. Preferably, weakly dissociated acids, for example formic acid, acetic acid, lactic acid, glycollic acid or oxalic acid, are used. However, there may also be used medium strength acids, or even mixtures of various acids. when using strong acids, care is to be taken, for example, by adding buffer salts, that the evolution of chlorine dioxide does not proceed too rapidly. Furthermore, acid salts such as primary sodium phosphate or .primary sodium sulfate, or even buffer mixtures such as a mixture of a mineral acid with sodium acetate, can also be used to adjust pH.

When unhairing is carried out in an aqueous medium, the unhairing bath is generally used in a quantity of about 10-500% (referred to the weight of the soaked hides). The optimum quantity of unhairing bath depends above all on the manner in which unhairing is carried out. If unhairing is carried out in a pit, relatively large quantities of water are generally used, since the material to be unhaired must completely be covered by the unhairing solution. The quantity of bath depends in this case essentially on the size and form of the pit in relation to the amount of hides. If unhairing is carried out in a closed drum, which in practice is the most widely used method, a relatively small quantity of bath, namely about l-100%, preferably 30-70%, referred to the weight of the soaked hides, is sufficient.

For effecting unhairing according to the present invention, chloride dioxide is caused to act for several hours on hides or pelts, preferably at room temperature or at a slightly elevated temperature, either with or without movement of the hides. The temperature maintained during unhairing has an influence on the time necessary for complete unhairing. At very low temperatures, for example, at temperatures below about 8 C., the oxidation proceeds so slowly that the process is no longer economical. Operation at too elevated temperatures, for example, at temperatures above about 50 C., involves the risk that the collagen of the skin may be damaged, so that it is not recommended to use temperatures exceeding about 50 C. Therefore, in practice, the process is carried out at temperatures between about 10 C. and about 45 C.

The period of time of hlorine dioxide action on the hides or pelts necessary to effect unhairing furthermore depends on the concentration of the chlorine dioxide in the aqueous ba-th. If the concentration of chlorine dioxide is high, unhairing will be completed in a shorter period of time than with a low concentration. When the operation is carried out in a drum under the conditions preferably employed in practice, for example at temperatures of between about 25 C., and 38 C., with a quantity of bath of about 30-70% (referred to the weight of the soaked hides), a chlorine dioxide concentration of 2.5-5%

(referred to the weight of the soaked hides), and an average number of revolutions of the drum of about 5-12 revolutions per minute, it can generally be assumed that unhairing is complete after about to 30 hours, depending on the type of the hides and pelts to be unhaired. In practice, the operational conditions, i.e. temperature, quantity of water, quantity of chlorine dioxide and number of revolutions of the drum, will generally be so adjusted that the treating period is within an economically acceptable range of 20 hours. In general, a minimum period of treatment of at least about 7 hours is necessary to achieve complete unhairing.

The above values apply to the ordinary conditions of practice. These values, however, are not intended to limit the invention, since in view of the varying nature of natural products it is impossible to establish generally valid operational conditions for their treatment. These conditions must be adapted to the requirements of any particular case. That the above values do not represent a limitation is evident from the fact that, for example, an excessively long treatment may be used and does not result in a damage of the hides and pelts thus treated. However, such an excessively long treatment exceeding the period of time necessary for complete unhairing most- 1y does not bring any advantage, and, therefore, will generally not be employed for reasons of economy.

For effecting complete unhairing, it is necessary to employ a certain minimum quantity of chlorine dioxide, referred to the weight of the hair to be removed. Besides this, the necessary quantity of chlorine dioxide furthermore depends on the goods to liquor ratio and the manner in which unhairing is carried out. When the operation is carried out in a drum with an average quantity of bath of about 30-70% (referred to the weight of the soaked hides), the minimum quantity of chlorine dioxide is in the range of about l-2 kg. of chlorine dioxide per kg. of hair to be removed, i.e. for hides and pelts having hair in a quantity of, for example 1% by weight, about 1-2 kg. of chlorine dioxide are used per kg. of hides and pelts. In general, there is no upper limit regarding the quantity of chlorine dioxide to be employed. An excessively high concentration of chlorine "dioxide, too, has no deteriorative effect on the hides to be treated. An excessively high concentration of chlorine dioxide in the bath is only insofar of disadvantage in practice as it may result in accumulations of unutilized chlorine dioxide in the space above the bath. These accumulations, when admixed with the air in the space above the bath, may imply the risk of an explosion. Normally, no chlorine dioxide or only an insignificant quantity of chlorine dioxide passes into the space above the bath, since the chlorine dioxide is consumed almost completely during the reaction in the bath, if the operation is carried out under adequate conditions. Apart from the above, economic reasons stand against the use of such an excessively large amount and concentration of chlorine dioxide.

If the unhairing according to the process of the present invention is carried out in aqueous solutions containing substances which yield chlorine dioxide, it will generally be sufficient to use these substances in quantities of about 1-10%, referred to the weight of the crude hides or pelts. This indication of the quantity, however, should not be considered as a limitation. It is quite possible to use larger quantities, but for reasons of economy the quantity of substance giving off chlorine dioxide will be kept at a minimum.

Furthermore, it is also possible to carry out unhairing according to the present invention by the so-called Schwdde process, in which a thick aqueous paste containing the chlorine dioxide yielding substance is applied to the hide or skin. In this case, larger quantities of up to about 50%, or even more, of chlorine dioxide yielding substance (referred to the weight of the hides and pelts) may be used.

Following the treatment with chlorine dioxide according to the present invention, the loosened hairs or scales are scraped off the treated skins or hides in the usual manner either manually or mechanically by means of an unhairing or scraping machine. When operating in a drum using the process of the present invention, the motion imparted to the skins is itself sufiicient to remove the hair. Even if the exact nature of the hides and pelts treated according to the present invention is not fully known, it has been found that the so treated hides and pelts are quite different in their nature from hides and pelts obtained by prior art unhairing processes.

In contradistinction to chemical liming processes, the oxidative unhairing process of the present invention does not cause swelling of the skin substance, apparently since it is carried out in the acid pH-range. A further advantage of the process of the invention over the known unhairing methods is that completely clean, pure white pelts are obtained even when spotted hides are unhaired, because the chlorine dioxide simultaneously has a bleaching action and destroys the hair pigment contained in the s In.

The action of the chlorine dioxide in the process of the present invention is believed due to the oxidation of the cystine-sulfur bridges of the keratin of the hair,

scales and epidermis to water-soluble keratin-sulfonic acids according to the following formulae:

4 keratin-S-S-keratin+l C10 +4 li o- 8 keratin-SO H-l- Cl,

The chlorine set free by reduction of chlorine dioxide is bound irreversibly by the collagen and thereby apparently causes chlorine pre-tanning of the external portions of the hides. This chlorine pro-tanning, produced simultaneously in the unhairing process of the present invention, additionally strengthens the grain.

A further advantage of the present invention is that, in unhairing with chlorine dioxide, a decomposition of transversely cross-linked collagen fibers is also caused. This in turn results in a desirable loosening of the structure of the fibers. Furthermore, the free chlorine liberated in the unhairing process of the present invention causes chlorination and saponification of triglyceridic fats. Assisted by the keratin-sulfonic acids contained in the unhairing bath, the chlorinated fats emulsify in the bath and are thus removed to a large extent from the hide. Hence, when operating by the unhairing process of the invention, it is possible to omit the step of degrading the skin by means of a saturated calcium hydroxide solution, usually carried out after dehairing, and of degrading by bating enzymes. The

multistage operation, heretofore necessary which included unhairing, liming, deliming and bating, can now be replaced by a single operation. Moreover, subse quent pickling may be omitted and chrome tanning can be carried out directly after the oxidative unhairing treatment. For the latter purpose, a portion of the weakly acidic unhairing solution can be used. Because of the keratin components therein, the solution stabilizes the chrome tanning agents and thus permits mild tannings and gives the leather improved fullness. The manufarture of leather is thus substantially simplified by the application of the new unhairing process. Oxidative unhairing according to the present invention permits the saving of labor and a, paratus and, moreover, it permits automation of the individual process steps in the manufacture of leather, starting from soaking up to tanning, without it being necessary to touch the skins after they have been introduced into the drum.

The process of the present invention can be applied to all animal hides and pelts. It is not limited to the unhairing of hides and pelts of any particular kind, and can be used for dehairing the hides or pelts of haired animals such as cattle. goats, stags, buffaloes, antelope, or sheep. It is also suitable for removing other keratincontaining parts of animal hides, for example, for removing scales from reptile skins.

The following examples illustrate the invention, but they are not intended to limit it thereto:

Example I A black and white spotted bovine hide having a soaked weight of 26 kg. was treated, after soaking in a pit, for 20 hours with a solution of 1.25 kg. of sodium chlorite and 0.5 kg. of glycollic acid in 100 l. of water and adjusted to pH 3.3-3.5 by means of about 0.5 kg. of 57% aqueous glycollic acid.

After this treatment, the hair was heavily felted and completely discolored. It could then be easily removed mechanically from the hide by an unhairing machine. The thus unhaired hide was pure white and did not contain any residues of scurf, sebaceous glands, urine stains, or hair pigment, as were found on hides simultaneously treated according to the lime-sulfide process using calcium hydroxide and sodium hydrogen sulfide. The pelt dehaired with sodium chlorite could be directly chrometanned without pickling and gave, after dyeing and stuffing, a leather having a uniform coloration and closely fixed grain.

One-half of a calf hide having a soaked weight of 2.3 kg. was drummed for 15 hours at 35 C. with a bath containing 3 l. of water and 0.3 kg. of a 30% aqueous sodium chlorite solution. The bath had been adjusted to pH. 3.2-3.5 by the addition of 0.1 kg. of a 57% aqueous glycollic acid solution; For comparison, the second half of the calf hide was drummed with a conventional limesulfide solution.

Whereas on the conventionally limed hide the zones of pigmented hair could still be seen and the fat-wrinkles were distinctly protruding. the hide unhaircd with the chlorite bath was pure white and smooth and did not show protruding fatwrinkles, even without mcclmniczil unhairing on an unhairing machine as is otherwise necessary.

Example 3 The separated neck of a heavy bulls hide was placed into a pit charged with 350% of water (referred to the weight of the skin) having a temperature of 25 C. The bath was adjusted to pH 7.5. Then, gaseous chlorine dioxide was introduced into the bath through a distributor tube for 25 hours. The chlorine dioxide was produced from a sodium chlorite solution by the addition of acid and transferred by means of a continuous stream of gaseous nitrogen serving as an inert diluent. In the same manner, a streain of a gaseous mixture of chlorine dioxide and carbon dioxide, produced by the action of oxalic acid on sodium chlorate, could be used. The bleaching action of the chlorine dioxide was already evident after a period of action of 3 hours. After 7 hours, the first separation of the hair from the hide could be observed. After 25 hours, all hair not yet destroyed could be easily removed by means of an unhairing machine. The sulfonated hair can be dissolved by adding a 10% sodium carbonate solution. A well degreased pelt was obtained, the hair pigments of which were completely dccolorcd, and which showed no protruding fat-wrinkles.

Example 4 kg. of bovine hides, fleshed as usual in the slaughter-house, in salted condition, were introduced into a tanning drum revolving at about 6 revolutions per minute. The hides were soaked first for 4 hours in 300 l. of water at 15 C., to which had been added 1.2 kg. of concentrated sulfuric acid.

The soaking solution was then discharged and 50 l. of water at 15 C. and 10 kg. of 30% aqueous sodium chlorite were added. The hides were then drummed for 2 minutes with this solution. Then, -1 kg. of lactic acid was added slowly through the tubular axle of the drum and the pH-value of the unhairing bath was adjusted to a value in the range of 3 and 3.5 by the addition of a 30% sulfuric acid. After a drumming time of 20 hours, unhairing was completed and the unhairing solution was discharged. The pickle pelt thus obtained was free of neck and fat-wrinkles which this type of hide usually shows.

50 l. of water and 5% of chromic oxide (referred to the weight of the crude hides), in the form of the commercially available powdered chromium tanning salt, were then introduced into the drum and the hides wcrc drummed for 5 hours. The tanning agent was then neutralized during drumming in the course of 5-6 hours b the slow addition of about 1-1.5 kg. of sodium bicarbonate.

The above-described operation permits fully automatic operation in the manufacture of leather, so that soaking, unhairing and tanning can be carried out within 36 hours without it being necessary to handle the hides after they have been introduced into the drum.

If tanning with vegetable or synthetic tanning agents is desired, the bovine hides in the above-described method of operation are dcpickled. al'tcr unhairing and discharge of the uuhairiag bath. by the addition of 1.5 kg. of powdered sodium thiosulfate and drumming for 30 minutes without addition of water. and then rinsed for 15 minutes v. ith water. Tanning with vegetable or synthetic tanning agents can then directly be carried out. There is no risk of over-tanning. in which the tanning agent precipitates too rapidly on the surface of the pelt and thus forms 'a diffusion-inhibiting layer which prevents the tanning agent from further penetration into the pelt.

The amotnt of water required in the above-described method of operation. i.c. from soaking to tanning, or of waste water obtained in this operation, is in the range of only 4-5 m. per 1000 kg. of hides. in addition, the

I (l.. A. Kuutzel, tterhereichvuiisehes Teschenhueh. Dresden. LeipzlH.

nth Edition, 1955. pages 4?- 49.

The waste water is biologically harmless, as is evidenced by the fact that when it is added, for example, to a fishpond in a quantity of 150 g./l., it does not harm the fish or affect the plants therein.

The amount of waste water in the hitherto used limesodium sulfide unhairing process for the same quantity of hides is in the range of at least about 40-50 m5. In addition. the removal of the waste water of the limesulfide process involves the ditficulties described in the introductory part of this specification.

Example 5 100 kg. of sheep skins were soaked in the usual manner and sheared on a wet--ncaring machine to a length of hair of a out 0.3 cm. 'lhe wool obtained could be sold as not chemically treated sheared wool. The shorn hides \tere drummed in a drum containing 100 l. of water, kg. of a 30% aqueous sodium chlorite solution and 2 kg. of 60% aqueous acetic acid. The pH-value of the unhairing liquor was from 3.3 to 3.5. During drumming, the hides were unhaired and. simultaneously, the chlorine set free caused chlorination and saponification of the natural fat which was thus easily emulsified and separated from the pelt.

After drumming for 18 hours, pickled pelts were obtained which were almost free from natural fat. except for a residual fat content of about 1%, and which. after addition of about 01-05% (referred to the weight of the pelt) of a commercial preserving agent such as chloroacetamide or pentachlorophenol sodium, for example, could be stored and shipped in untanned condition. After a storing period of 1 year. for exampic, no signs of decomposition could be observed. Besides. the hides treated in this manner showed a strengthening in the usually slightly spongy fibrous structure which had not been attained in sheep hides before. The structural strengthening was apparently the result of a kind of pretanning of the pelt caused by the action of the chlorine which was set free during unhairing.

The method of operation described above can be applied with equally good results to sheep hides which have been unhaired enzymatically or dewoollcd by the so-called sweating process (cf. Dr. H. Gnamm, Fachbuch fi'tr die Lederindustrie, 3rd edition, Wissenschat'tl. Verlagsgesellschatt, Stuttgart. 1946, pages 90-92). Short-hair and natural fats are completely removed and at the same time, the collagen fiber structure is strengthened.

Example 6 10 kg. of dried snake hides (python) were soaked in the usual manner and then drtnnmed for 25 hours in a drutn containing 20 l. of water of 20 C., 1.5 kg. of a aqueous sodium ehlorite solution and 0.25 kg. of phosphoric acid. The pH-value of the liquor was 3.0- 3.3. Head parts and tail parts were tied together in order to avoid entanglement.

By this treatment, the keratin-containing scales of the hides were removed completely from the collagen fiber structure. At the same time about of the design of the hides was bleached off. For the final bleaching the hides were drummed for 8 hours at 20 C. with a soltttion of 0.1 kg. of potassium permanganate in 20 l. of water, then after treated in the drttm for 1 hour with a solution of 0.15 kg. of sodium bisulfite in 20 l. of water in orderto remove the manganese dioxide formed, and subsequently rinsed with water. The pelt obtained in this manner was of a pure white and could be treated directly with tawing agents.

Example 7 A calfs hide soaked in the usual manner was hung in a wet state in a chamber which could be closed hermetically and which was provided with a gas inlet and outlet tube. The chamber was closed and the air contained in this chamber was displaced by the introduction of nitrogen. Then. a uniform stream of a mixture of gases, consisting of nitrogen and chlorine dioxide in equal parts by volume, was passed through for 2:) hours at 20 C. After a period of action of 1 hour, a strong bleaching effect could be observed. After 20 hours, the gas mixture in the chamber was displaced by the introduction of pure nitrogen. The discolored hair of the calfs hide could then be easily removed from the hide manually or by means of an unhairing machine. The pelt thus obtained could be treated directly with vegetable tanning agents or, after pickling. with chrome tanning agents.

1 claim:

1. A method for removing the keratin-containing parts 01' animal hides and pelts and the keratin-containing parts adhering to such hides and pelts, which comprises contacting said hides and pelts with chlorine dioxide in an aqueous bath for a time sutficient to loosen said keratincontaining parts, said aqueous bath having a pH up to pH 6 and comprising from about 1 to 10 percent. by weight of the hides and pelt treated, of an alkali metal chlorite. said chlorine dioxide being generated in situ in said aqueous bath from said alkali metal chlorite.

2. A method as in claim 1 wherein said hides and pelts are treated in said aqueous bath for at least 7 hours at a temperature between about 10 C. and about C.

3. A method for removing the keratin-containing parts of animal hides and pelts and the keratin-containing parts adhering to such hides and pelts which comprises contacting said hides and pelts with chlorine dioxide in an aqueous bath for a time sufiicient to loosen said keratincontaining parts. said aqueous bath having a pH up to pH 6 and said chlorine dioxide being introduced as a gas into said aqueous bath from a source outside said bath in an amount of at least 1-2 kilograms per kilogram of keratin-containing parts to be removed. t

4. The method as in claim 3 wherein said hides and pelts are contacted with chlorine dioxide at a temperature below C.

9 10 of at least 1-2 kilograms per kilogram of keratin-con- 935,991 11/1955 Germany. mining parts to be removed. 749,769 5/1956 Great Britain.

OTHER REFERENCES References Cited by the Examiner McLaughlin ct al.: Chemistry of Leather Manufacture,

UNITED STATES PATENTS 5 1945, pages 186-188, by Reinhold Pub. Corp. 3,097,049 7/1963 Maybeck ct al. 8-108 Speakman: J. of Textile Institute, July 1941, pp. T93- FOREIGN PATENTS 118,721 8/1944 Australia. NORMAN G. TORCHIN, Primary Examiner. 

1. A METHOD FOR REMOVING THE KERATIN-CONTAINING PARTS OF ANIMAL HIDES AND PELTS AND THE KERTAIN-CONTAINING PARTS ADHERING TO SUCH HIDES AND PELTS, WHICH COMPRISES CONTACTING SAID HIDES AND PELTS WITH CHLORINE DIOXIDE IN AN AQUEOUS BATH FOR A TIME SUFFICIENT TO LOOSEN SAID KERTAINCONTAINING PARTS, SAID AQUEOUS BATH HAVING A PH UP TO PH 6 AND COMPRISING FROM ABOUT 1 TO 10 PERCENT, BY WEIGHT OF THE HIDES AND PELTS TREATED, OF AN ALKALI METAL CHLORITE, SAID CHLORINE DIOXIDE BEING GENERATED IN SITU IN SAID AQUEOUS BATH FROM SAID ALKALI METAL CHLORITE. 